What could an admittedly awesome B-movie about alien shapeshifters taking over a small American town have to do with McCarthyism? A whole lot, but on an allegorical level.

The most successful horror movies reflect the fears and anxieties of the day. That's how they make money: by scaring people. The easiest way to scare people is to show them what they're already scared of. Invasion of the Body Snatchers played into fears of a vast and coordinated army of agents from a distant power who looked just like us but wanted to turn us into soulless monsters like Communists.